Stable, low-noise sources of ultra short pulses are important for a wide range of applications including ultrafast spectroscopy, multi-photon excitation microscopy, micromachining, harmonic generation, and the pumping parametric amplifiers and oscillators. Such sources are usually referred to by practitioners of the art as ultrafast lasers. The most common ultrafast lasers are solid-state lasers having a crystal gain-medium, such as titanium-doped sapphire (Ti:sapphire), with a broad gain-bandwidth.
Fiber lasers offer an attractive alternative to solid-state ultrafast lasers. Such lasers provide an ability to operate in a wide range of pulse-repetition frequency, for example, between about 10 megahertz (MHz) and 400 MHz and can be contained in a compact package. Compared to solid-state crystal lasers, fiber lasers have high intra-cavity dispersion. This is because most of the resonator (cavity) optical path of the laser is in glass (the fiber) and not in air. It is well known that the less the intra-cavity dispersion, the shorter the pulses that can be obtained.
There are a few methods for reducing intra-cavity dispersion in fiber lasers. One uses chirped fiber Bragg gratings (CFBG). Another uses a compressor and stretcher arrangement based on bulk diffraction gratings. Yet another method uses a photonic-bandgap fiber or other specialty fiber with a specially designed dispersion-profile to compensate dispersion provided by a gain-fiber and intra-cavity components.
Chirped fiber Bragg gratings (CFBGs) are often used for generating picosecond pulses from an all-fiber cavity when fiber dispersion is not completely compensated. At low dispersion, however, for example less than 1 picosecond per nanometer (ps/nm), a CFBG typically has a low reflectivity (<40%). Such a low reflectivity makes it difficult to use such gratings in mode-locked fiber-lasers wherein reflectivity required for mode-locking is typically above 50%. Mode-locked fiber lasers are often used when pulses shorter than 1 picosecond (ps) are required.
A compressor arrangement based on diffraction gratings provides adjustable dispersion, which can be tuned to exactly compensate intra-cavity dispersion. An example 10 of such a compressor arrangement is depicted in FIG. 1. A compressor 10 includes essentially identical diffraction gratings 12A and 12B arranged spaced apart and parallel to each other in combination with a mirror 14 comprising a multilayer reflective coating 16 on a substrate 18.
Here, an input pulse PIn has a spectral bandwidth λ3 minus λ1, and a center wavelength λ2. It is assumed that, as a result of intra-cavity dispersion, shorter wavelengths such as λ1 have been delayed more than longer wavelengths such as λ3 thereby increasing the duration (length) of the pulse. Grating 12A diffracts the pulse wavelengths at different angles with longer wavelengths such as λ3 diffracted at a greater angle than shorter wavelengths such as λ1. Grating 12B directs the variously diffracted wavelengths along parallel paths to be incident mirror 14. Mirror 14 reflects the wavelengths back along their incident paths to recombine, on the path of the input pulse, as an output pulse Pout. As the paths of the longer wavelengths are longer than those of the shorter wavelengths, the shorter wavelengths “catch-up” with the longer wavelengths sufficiently that output pulse has a shorter duration that the input pulse.
With the mirror of a “grating pair” pulse-compressor such as compressor 10 used an end-mirror of a mode-locked fiber-laser pulses shorter thanl ps have been obtained. Such a compressor however has certain drawbacks. One drawback is that control of the lasing wavelength in the laser cavity with a compressor is difficult as spectral selectivity of the compressor is very low. By way of example FIG. 2 schematically illustrates typical measured diffraction efficiency of a transmission grating such as gratings 12A and 12B. It can be seen that diffraction efficiency varies by only a few percent over a spectral (wavelength) range between about 1000 nm and 1080 nm. Accordingly, the prior art compressor of FIG. 1 could at best be described as weakly wavelength selective in this range, which represents about the full gain-range (emission range) of an ytterbium (Yb) doped gain-fiber.
FIG. 3 schematically illustrates a typical prior-art arrangement 20 a mode-locked fiber laser including a grating-pair compressor such as compressor 10 of FIG. 1. Laser 20 has a resonant cavity formed between mirror 14 of compressor 10 and a saturable Bragg reflector (SBR) 22, which provides passive mode-locking of the laser. The resonant cavity (resonator) includes an active (doped) fiber 26 and through-fibers 32 and 36 of couplers 28 and 30 spliced to the active fiber. SBR 22 is formed from a saturable absorber and a Bragg reflector. One surface of the SBR 22 is mounted to a substrate 26 and the opposed surface is butt-coupled to fiber 36. A lens 40 collimates radiation from the fiber portion of the resonator before the radiation enters compressor 10. Lens 40 focuses radiation from the compressor back into the fiber portion of the resonator. A half-wave plate 42 adjusts polarization of returning radiation to maximize transmission through the gratings 12B and 12A. Coupler 30 is a wavelength-division multiplexing (WDM) coupler coupling pump radiation into the fiber portion of the resonator via a fiber 38. Coupler 28 is a fractional coupler which couples a fraction of circulating radiation out of the resonator, as mode-locked output pulses, via a fiber 34.
FIG. 4 schematically illustrates the absorption (solid curve) and emission (dashed curve) spectra over a wavelength range from about 850 nm to 1150 nm. It can be seen that the emission-curve has a strong narrow peak centered at about 975 nm partially overlapping a peak in the absorption spectrum centered at a wavelength of about 980 nm which is the usually preferred pump-wavelength for a Yb-doped fiber. At longer wavelengths, the emission curve varies relatively strongly with a peak gain between about 1035 nm and about 1040 nm. Because of the relatively weak spectral selectivity of compressor 10, and in the absence of any other spectral selective device in the resonator, the gain-curve would dominate the wavelength selection process and the resonator (pumped by 980 nm radiation) would oscillate in the 1035 nm to 1040 nm peak-gain region.
Apart from the lack of spectral selectivity another drawback of the grating-pair compressor is that circulating radiation makes forward and reverse passes through each grating. Even given an efficiency of about 95%, as indicated in FIG. 2, the four passes would introduce resonator losses of about 20%. While active fiber 26 has high gain and the resonator can tolerate relatively high losses, such losses detract from overall efficiency of a fiber laser. Yet another drawback is the high cost of the diffraction gratings, which are by far the most expensive components in the laser cavity. Including two such gratings makes what would be a relatively inexpensive laser quite costly.
There is a need for an intra-cavity compressor arrangement which has sufficiently high spectral selectivity to determine a lasing wavelength within the gain-bandwidth of a gain-fiber but does not require two diffraction gratings. Such a compressor could enable the building of low-cost femtosecond laser systems with controllable lasing wavelength, which could expand the range of applications for such systems.